This invention is generally related to micro-electro mechanical systems (MEMS) especially MEMS switches, and more specifically, to an inductive MEMS switch utilizing inductive coupling and decoupling, and which is fully compatible with standard CMOS manufacturing materials and processes.
Switching operations are a fundamental part of many electrical, mechanical and electro mechanical applications. MEMS switches have drawn considerable interest over the last few years. Products using MEMS switch technology are widespread in biomedical, aerospace and communications systems.
MEMS switches have been manufactured using various configurations, they are electrostatically controlled beams that make metal-to-metal contact or a similar structure that uses a dielectric stop to form a capacitive switch. A common feature that characterizes the device is that it is provided with at least one moving element contacting another to complete the circuit.
In order to better understand the present invention, a conventional MEMS switch will now be described with reference to FIG. 1, showing a cross-section view of a MEMS switch having both ends of a deformable beam 5 anchored on dielectric 2. The lowest level consists of a dielectric material 1 consisting of conductive elements 3 and 4 which are used to connect or form the various electrical components of the device. Conductors referenced by numerals 3 and 6 are used to provide an operating voltage potential that causes the beam to bend (or deform). Conductor 4, which conducts an electrical signal, contacts the deformable beam when the MEMO switch is in operation. FIG. 2 shows a top-down view of the same conventional switch.
In a typical implementation of a conventional MEMO switch, the contact beam is formed by depositing polysilicon over a dielectric made of, e.g., SiO2. The surrounding material is etched away leaving a raised structure that is attached to silicon beam 5. The contact element 6, anchored at one end on silicon beam 5 is suspended at its other end above conductors 3 and 4, and is preferably made of polysilicon. Subsequently, the device is subjected to electroless plating, usually of gold, that adheres to the polysilicon to complete the fabrication of conductive elements 3, 4 and 6.
The switch is operated by providing a potential difference between contact beam 6 and electrode 3. This voltage generates an electrostatic attraction that brings beam 6 in contact with electrode 4, thus closing the switch. The twist imparted to the anchored beam 5 is used to restore the contact 6 to its open position once the control voltage potential is dropped.
Generally, all conventional MEMS switches rely upon physical contact, especially metal-to-metal contact to perform the switching operation. This leads to many reliability problems related to arcing, material transfer, micro-welding, station, and the like. It is well known in the art that most of these switches become less reliable at higher frequencies. Some of the metallurgies used, such as gold, that are commonly used in an attempt to alleviate these problems are not compatible with standard CMOS fabrication. The inductive MEMS switch of the present invention which will be described hereinafter lends itself to be operated by any number of well known MEMS actuators.
Examples of MEMS actuators can be found at the Sandia National Laboratory web site (www.sandia.gov), or in several MEMS patents related to actuators such as U.S. Pat. No. 6,328,903, George E. Vernon, Sr., xe2x80x9cSurface-Micromachined Chain for Use in Micro-Mechanical Structuresxe2x80x9d, issued Dec. 11, 2001. Other patents specifically directed to comb drive systems described hereinafter are to be found, for instance, in U.S. Pat. No. 5,998,906, Jerman et al., xe2x80x9cElectrostatic Microactuator and Method for Use Thereofxe2x80x9d, issued Dec. 7, 1999.
In conventional MEMS switches, as described, for instance, in U.S. Pat. No. 6,074,890, Yao et al., Method of Fabricating Suspended Single Crystal Silicon MEMS Devicesxe2x80x9d, issued Jun. 13, 2000, and further described in IEEE Microwave issued December 2001, typically, at least one electrode in the switching circuit has a DC potential applied as part of the electrostatic actuation. Thus, a distinct need exists to separate the drive system from the switching circuit such that no DC control voltage is applied to at least one contact in order to perform electrostatic actuation.
Accordingly, it is an object of the invention to provide an inductive MEMS switch based on inductive coupling and decoupling of electrical signals.
It is another object to provide an inductive MEMS switch that isolates the control signal from the switched signals by separating the path of the switched signal from the control circuit used to operate the device.
It is still another object to provide an inductive MEMS switch having an off-state isolation that surpasses a conventional switch in the off-state, and which are typically limited to provide isolation of about 50 dB at 6 GHz.
It is a further object to provide an inductive MEMS switch that may be configured in a variety of multi-pole, multi-throw arrangements and which is controlled by any number of MEMS linear or rotary drive systems.
It is yet another object to provide an inductive MEMS switch that can reliably perform xe2x80x9chot-switchingxe2x80x9d, namely, switching while operating under nominal power levels. Switching can be achieved at 1 watt, 5 watts up to whatever value the remaining part of the circuit cansince the switching is non-contact and, thus, there is no arcing or welding of contacts.
It is still another object to provide an inductive MEMS switch that operates reliably with no DC potential or physical contact point in the signal path which can potentially lead to arcing, welding or material transfer and degradation.
It is still a further object to provide an inductive MEMS switch that increases its efficiency at higher frequencies, allowing the size of the coils to decrease when the frequency of the signal increases, the increase in efficiency being achieved by magnetic field coupling between the switch components, thus providing better insertion loss characteristics at higher frequencies without a corresponding decrease in isolation performance.
It is another object of the invention to provide a switch/transformer combination for achieve impedance matching. By selecting the inductance of each portion of the inductive switch appropriately, the input and output impedance of the switch can be adjusted independently. This adjustment allows for impedance matching and switching at the same time. A special configuration of the transformer can be utilized to create a single-ended to double-ended converter or balun (BAlanced-UNbalanced), providing both switching and signal conversion in a single device.
It is still a further object to provide an inductive MEMS switch that can be manufactured using CMOS compatible processes and materials.
In one aspect of the invention, switching of the signal is accomplished by inductive coupling and decoupling between stationary coils and movable coils. Switching occurs as the movable coils are or not aligned with respect to the stationary coils.
A four turn spiral inductor, with a metal thickness of 4 xcexcm, a turn width of 10 xcexcm, and an outer diameter of 150 xcexcm, configured as one element of the switch, is magnetically coupled to another similar spiral, directly above or below, yielding a coupling coefficient of about 0.85. When these spirals are configured as described, a closed-switch insertion loss of 6.6 dB and a opened-switch isolation of 65 dB is achieved at 13 Ghz. This yields an excellent on-off switch ratio tuned to frequencies below 13 Ghz by adding an external tuning capacitor between the two ports of the switch. Similarly, a one and a half turn spiral inductor, with a metal thickness of 4 xcexcm, a turn width of 10 xcexcm, and an outer diameter of 150 xcexcm, configured as one element of the switch, is magnetically coupled to another similar spiral, directly above or below, yielding a coupling coefficient of about 0.85. When the spirals are configured as such, a closed-switch insertion loss of 10 dB and an opened-switch isolation of 60 dB is achieved at 25 Ghz. This yields an excellent on-off switch ratio tuned to frequencies below 25 Ghz by the addition of an external tuning capacitor between the two ports of the switch.
In another aspect of the invention, the present. MEMS switch solves problems known as stiction, arcing and welding of the switch contacts, all of which are eliminated because of a lack of physical contact between the switching elements. The coils are simply aligned in close proximity such that inductive coupling can transfer the signal between one and the other. In view of this characteristic, the MEMS switch can easily handle switching at full power (hot switching) and, clearly more power than a conventional MEMS switch.
Multiple switch configurations are realized by varying the number of stationary or movable coils, and/or by altering the coil geometric configuration of the coils and the corresponding displacement of the movable elements. Additionally, total isolation from the control signal and the switched signal path is possible since the drive circuit is totally independent of the switching circuit.